Organic matter and sediment properties determine in-lake variability of sediment CO<sub>2</sub> and CH<sub>4</sub> production and emissions of a small and shallow lake

Abstract

Abstract. Inland waters, particularly small and shallow lakes, are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, the spatial in-lake heterogeneity of CO2 and CH4 production processes and their drivers in the sediment remain poorly studied. We measured potential CO2 and CH4 production in slurry incubations from 12 sites within the small and shallow crater lake Windsborn in Germany, as well as fluxes at the water–atmosphere interface of intact sediment core incubations from four sites. Production rates were highly variable and ranged from 7.2 to 38.5 µmol CO2 gC−1 d−1 and from 5.4 to 33.5 µmol CH4 gC−1 d−1. Fluxes ranged from 4.5 to 26.9 mmol CO2 m−2 d−1 and from 0 to 9.8 mmol CH4 m−2 d−1. Both CO2 and CH4 production rates and the CH4 fluxes exhibited a significant and negative correlation (p<0.05, ρ<−0.6) with a prevalence of recalcitrant organic matter (OM) compounds in the sediment as identified by Fourier-transformed infrared spectroscopy. The carbon / nitrogen ratio exhibited a significant negative correlation (p<0.01, ρ=-0.88) with CH4 fluxes but not with production rates or CO2 fluxes. The availability of inorganic (nitrate, sulfate, ferric iron) and organic (humic acids) electron acceptors failed to explain differences in CH4 production rates, assuming a competitive suppression, but observed non-methanogenic CO2 production could be explained up to 91 % by prevalent electron acceptors. We did not find clear relationships between OM quality, the thermodynamics of methanogenic pathways (acetoclastic vs. hydrogenotrophic) and electron-accepting capacity of the OM. Differences in CH4 fluxes were interestingly to a large part explained by grain size distribution (p<0.05, ρ=±0.65). Surprisingly though, sediment gas storage, potential production rates and water–atmosphere fluxes were decoupled from each other and did not show any correlations. Our results show that within a small lake, sediment CO2 and CH4 production shows significant spatial variability which is mainly driven by spatial differences in the degradability of the sediment OM. We highlight that studies on production rates and sediment quality need to be interpreted with care, though, in terms of deducing emission rates and patterns as approaches based on production rates only neglect physical sediment properties and production and oxidation processes in the water column as major controls on actual emissions.

Whole‐lake methane emissions from two temperate shallow lakes with fluctuating water levels: Relevance of spatiotemporal patterns

“…According to a recent study on sediment properties in Lake Windsborn (Praetzel et al 2020), the higher C : N ratios toward the shore in Lake Windsborn indicate an input of allochthonous (terrestrial) material near the shore and sedimentation of autochthonous (aquatic) organic matter in the lake center. It is known that allochthonous material is more difficult to decompose than material of autochthonous origin (Sobek et al 2012; Grasset et al 2018).…”

“…The samples were freeze dried, ground, and analyzed for total carbon (C), nitrogen (N) using isotope‐ratio mass spectrometry (Eurovector EA3000 coupled with Nu Instruments Nu Horizon, Hekatech, Wegberg, Germany). Details of the method were described in Praetzel et al (2020). The proportion of inorganic carbon was less than 0.1% and was therefore neglected.…”

Temperature and sediment properties drive spatiotemporal variability of methane ebullition in a small and shallow temperate lake

“…depth 1.7 m) polymictic crater lake in the Eifel uplands in Southwest Germany. It is nearly circular and surrounded by a 20–30‐m‐high rampart, resulting in a very small catchment area (~ 8 ha) and the absence of surficial inflows and outflows (Praetzel et al 2020 and references herein). During 2017 and 2018, Lake Windsborn was in a mesotrophic state with average measured pH values of 6.8 ± 0.8, conductivity of 19.4 ± 1.7 μ S cm −1 , 9.7 ± 0.9 mg L −1 of oxygen, 13.7 ± 2.3 mg L −1 of dissolved organic carbon, 1.0 ± 1.9 mg L −1 of total nitrogen, and 0.1 ± 0.1 mg L −1 of total phosphorous (Praetzel et al 2020).…”

“…In addition, OM in the lake center could have undergone intensified decomposition during resuspension and focusing of sediment particles (Mackay et al 2012). Although CH 4 production in incubations has been shown to strongly correlate with sediment OM quality in Lake Windsborn (Praetzel et al 2020), other sediment characteristics might also be important for bubble formation, growth and subsequent ebullitive release from the sediment in situ or in intact sediments. This might be responsible for the observed discrepancy between CH 4 production in the sediment and ebullition.…”

“…Raw data related to this article is available from PANGAEA database (Praetzel et al 2020, https://doi.pangaea.de/10.1594/ PANGAEA.917518) published under license CC-BY-4.0.…”